Biochemistry · Molecule
Bilirubin !
Bilirubin is the yellow pigment left when heme is broken down; when it builds up, it turns skin and eyes yellow (jaundice).
Updated 2026-11-01
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Key facts
- Normal total bilirubin in adults: up to about 1.2 mg/dL; direct (conjugated) below 0.3 mg/dL.
- Jaundice becomes visible when total bilirubin exceeds about 2–3 mg/dL; first in the sclerae.
- Unconjugated (indirect): fat-soluble, travels on albumin, does not appear in urine and crosses the blood–brain barrier.
- Conjugated (direct): water-soluble; when it builds up in blood it appears in urine (dark urine).
- Formula
- C₃₃H₃₆N₄O₆
- PubChem
- CID 5280352
Structure
- A linear tetrapyrrole (the heme ring opened); internal hydrogen bonds fold it and hide its polar groups, which is why it is insoluble.
- Blue light (phototherapy) converts it into isomers that cannot fold and are excreted without conjugation.
Clinical notes
- Indirect hyperbilirubinemia: hemolysis, Gilbert, Crigler-Najjar, neonatal jaundice. Direct: hepatitis, cholestasis, biliary obstruction, Dubin-Johnson, Rotor.
- Free bilirubin in newborns can stain the basal ganglia (kernicterus); some drugs (sulfonamides, ceftriaxone) displace it from albumin.
- Delta bilirubin, covalently bound to albumin, lasts for weeks and explains why jaundice fades slowly after an obstruction is relieved.
- Pigment (black) gallstones are made of calcium bilirubinate and are typical of chronic hemolysis.
Pathways
Related conditions
- Gilbert syndrome
- Crigler-Najjar syndrome
- Dubin-Johnson syndrome
- Rotor syndrome
- Neonatal jaundice
- Gallstones (cholelithiasis)
- Viral hepatitis
Related
- Heme
- Heme oxygenase
- Biliverdin reductase
- UGT1A1 (bilirubin UDP-glucuronosyltransferase)
- Hemoglobin and 2,3-BPG
Test yourself
Think of the answer, then tap the question to see it.
Which bilirubin fraction can appear in urine?
Only conjugated (direct) bilirubin, because it is water-soluble and not bound to albumin.
Why can unconjugated bilirubin cause kernicterus?
It is fat-soluble; the free fraction crosses the immature blood–brain barrier and deposits in the basal ganglia.
How does phototherapy act on bilirubin?
Blue light turns it into more soluble isomers that leave in bile and urine without conjugation.
At what level does jaundice usually become visible?
When total bilirubin exceeds about 2–3 mg/dL, first in the sclerae.
Molecules are shown in their neutral form, as the databases deposit them; in the cell the acids and phosphates are ionised.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: Bilirubin (CID 5280352) (PubChem, NCBI, 2026) · Public domain; Bilirubin blood test: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Jaundice | Icterus | MedlinePlus (MedlinePlus, US National Library of Medicine, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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